Thin film transistors (TFTs) are utilized in various devices, such as a liquid crystal displays, static random access memories and in nonvolatile memories. U.S. Pat. Nos. 5,572,046 and 5,383,149 also suggest using TFTs in a mask read only memory (mask ROM or MROM). Conventional TFTs have a structure that is similar to conventional bulk metal oxide semiconductor field effect transistors (MOSFETs), except that TFTs are formed in a semiconductor layer that is located above an insulating substrate, such as a glass substrate, or above a semiconductor substrate that is covered by an insulating layer.
The mask ROM derives its name from the fact that this nonvolatile memory is programmed or written using a custom mask during fabrication. In a conventional mask ROM fabrication process, a semi-finished array of transistors is covered by a photoresist layer. This photoresist layer is patterned using the custom photolithography mask to expose the channel regions of a first set of transistors while covering the channel regions of a second set of transistors of the array. The exposed channel regions are then rendered non-conductive by implanting the exposed channel regions of a first conductivity type with ions of the same conductivity type in order to raise the threshold voltage of the implanted transistors. Thus, the mask ROM array contains a first set of enabled transistors and a second set of disabled transistors.
Each bit of information in a mask ROM is stored by the presence or absence of a data path from a word (access) line to a bit (sense) line. The data path is eliminated by ensuring that no operational circuit element (i.e., transistor) joins a word and a bit line, such as by increasing the threshold voltage of selected transistors by the selective ion implantation. When a word line of a mask ROM is activated, the presence of a signal on the bit line will mean that a “1” is stored, whereas the absence of a signal will indicate that the bit location is storing a “o”, as discussed on pages 619-621 of S. Wolf, Silicon Processing for the VLSI Era, Vol. 2, Lattice Press, 1990, incorporated herein by reference. Mask ROM arrays may be implemented as NOR or NAND arrays, for example.
The state of the art mask ROM has an effective cell area of 4 F2, where F is the minimum feature size. This cell area is larger than desirable, and leads to a less than optimum cell density, which increases the cost of the mask ROM.